1. Field of the Invention
The present invention relates to an epoxy resin composition, and a prepreg and a printed wiring board using the epoxy resin composition, and more particularly to a halogen-free flame-retardant epoxy resin composition having improved processability and fracture toughness required for manufacturing a printed circuit board (PCB), and a prepreg impregnated with such a composition and a printed circuit board manufactured by using such a prepreg.
2. The Prior Arts
The use of a copolymer of styrene and maleic anhydride (SMA) as a curing agent for epoxy resin in an epoxy resin composition was described in U.S. Pat. No. 6,534,181 and TW 455613. A drawback to such epoxy resin compositions is that they have low Tg and low thermal stability, rendering them unsuitable for use in prepregs, which are applied in laminates for printed circuit boards (PCBs).
Prepregs are widely employed in the manufacture of laminates for printed circuit boards. For manufacturing a prepreg, in general, a substrate was impregnated with a varnish prepared by dissolving a thermosetting resin, such as epoxy resin, in a solvent, followed by partial curing of the resin, and such a substrate with impregnated and partially cured resin is commonly referred to as a prepreg. For manufacturing a printed circuit board, in general, it involves laminating a particular number of layers of the prepregs, and forming a metal foil additionally on at least one outermost layer, and forming a particular circuit pattern on the surface of the metal-clad laminate by etching the metal foil thereon.
However, processing prepregs into printed circuit boards usually involves their being cut down to size and laminated. Both these process steps make stringent demands on the resin with which the substrate is impregnated. For instance, the partially cured resin has to have sufficient sturdiness during the processing of prepregs into printed circuit boards.
In this connection resin compositions where the epoxy resin is cured with an anhydride-containing copolymer have the drawback of being too brittle to be processed as prepregs. For instance, it proves impossible to cut up such prepregs without a portion of the resin blowing about in the form of a large quantity of dry dust.
In the prior art, there were some methods for enhancing the toughness of the epoxy resins. Among which, the method for adding toughening agents was one effective method for toughening the epoxy resins. Toughness of the epoxy resin could be significantly increased with the addition of a thermoplastic resin having a molecular weight of larger than 5000, such as a phenoxy resin, or a carboxyl-terminated butadiene-acrylonitrile (CTBN) elastomer, which results in the decrease of the amount of resin dust during the processing of prepregs into printed circuit boards. However, adding toughening agents had an adverse effect on the heat resistance, dielectric properties, and bonding strength of the epoxy resins.
Therefore, there still exists a need for providing a resin composition where the problem of brittleness, which occurs when a copolymer of styrene and maleic anhydride is used as epoxy curing agent, can be prevented. Meanwhile, the resin composition based on epoxy resin cross-linked with styrene maleic anhydride copolymer (SMA) can have improved thermal, dielectric properties, and bonding strength.